So, after seeing Brave several times, it appears Young MacGuffin, who speaks only Doric (a dialect of English), has just three lines.
The first one is in the scene where the suitors are presented.
He says, "If he was a wee bit closer, I could lob a caber at him, ye ken."
If you know even a little about Scotland, then this one is pretty straight forward.
If you don't, I'll explain a wee bit here: Do you remember in the Highland Games scene when one of the men throws a a log that looks rather like a telephone pole? Well, that log is a called a"caber." The Caber Toss is a traditional Scottish game.
"Ken" is the Scot's word for "know."
Young MacGuffin's second line is right after Elinor turns into a bear and Fergus hears her roar and runs up the stairs to go bear hunting. I honestly can't understand it. To me, to sounded something like "Since yer sayin it I wasna anya say it myself. Og dae gahere wa a liɾle." (I'll go out there and watch a little?!) I'm missing words, possibly have it totally wrong...
Anyhow, just before his line, Lord MacGuffin says, "We haven't had desert yet."
His third line is in the scene where Merida walks into the room in the middle of the battle and then speaks with the lords. When she tells them she thinks they should all be able to find love for themselves the suitors each pipe in.
Young MacGuffin says, ""It's jist nae fair makin us ficht for the hand o the quine that disnae want any bit o it. Ken?" (Found on MetaFilter). The same person who posted this line also posted a translation: "It's just not fair making us fight for the hand of a girl who doesn't want anything to do with it. You know?"
The first and third lines and translations I found in a comment by gnuhavenpier on MetaFilter. (Scroll down)
The second one, I was trying to figure out by listening to the movie. Doric speakers, please help!!!
I wrote more about Young MacGuffin and Doric in my post: Brave -What is Young MacGuffin Saying?